A receiver of this type is described in the article entitled "Auto-adaptivite dans les faisceaux hertziens numeeriques" (i.e. Self-adaptivity in digital radio beams), by O. Toutan and O. de Luca, published in the journal "L'Onde Electrique", March 1986, Vol. 6, No. 2. This article specifies that the signal received by an antenna may be considered as being the sum of three signals: a direct signal and two delayed signals; and that for the purpose of combining the==signals, the transfer function of the propagation medium includes, in particular, a term which is sensitive to the frequency of the signal and which is responsible for distortion in the received signal.
This article therefore proposes combining the (composite) signals received from two vertically spaced-apart antennas receiving (individual) signals both of which have travelled through the same transmission medium and been subjected to identical delays, but which are at different relative phases. A diversity combiner sums the signals received by these two antennas after correcting their relative phase and adjusting their respective amplitudes. Phase correction is performed in an adjustable phase shifter placed on the path of one of the signals, and amplitude adjustment is performed by adjustable attenuators placed on the paths of both signals. A diversity control circuit including a microprocessor controls phase correction and level adjustment in such a manner as to ensure that the above-mentioned distortion term is at least greatly reduced, if not completely cancelled.
The embodiment described in this article relates to a heterodyne receiver including an intermediate frequency amplifier with automatic gain control (AGC), and signal analysis equipment including three "sensor" circuits each including a narrowband filter and respectively serving to evaluate the level of the signal in the middle and at the edges of the received spectrum. The diversity control circuit alternates between two operating modes depending on the power level of the wanted signal at intermediate frequency. When the received power level is sufficient, a distortion minimum is sought. The levels from the sensors are combined in order to provide the processor with a distortion value, with the processor acting alternately to adjust the phase shifter and to adjust the two attenuators in directions such that the distortion is reduced on each occasion until minimum distortion is reached.
Whenever the power of the wanted signal drops below a given threshold, then the processor is provided with the automatic gain control signal of the AGC amplifier, and the processor acts on the adjustments of the phase shifter and of the attenuators in order to reduce the AGC signal to a minimum, i.e. in order to increase the power of the wanted signal to a maximum. Whenever the appropriate conditions occur, the change-over between two these two modes of operation is abrupt.
The same article also mentions that an automatic time equalizer may be used in order to counter distortion due to multiple path propagation. In the example described, this equalizer operates at intermediate frequency on the basis of signals sampled in base band and processed by calculation circuits which provide coefficients for a correcting transversal filter. The use of such an equalizer is recommended for path lengths of more than 15 km, i.e. for most cases in practice (in particular when operating at 140 Mb/s with quadrature amplitude modulation (QAM)).
The present invention thus seeks, in receivers for space diversity radio transmission systems including a diversity combiner and a diversity control circuit, together with means for providing a distortion value and means for providing an insufficient power value, to simplify and generalize the prior art diversity control circuit.